This invention relates in general to all-reflective optical systems and in particular to an off-axis three-mirror anastigmat.
Reflective optical systems have long been the champion of the astronomical community, primarily because of their size, lightweight construction and broad spectral coverage. Slowly gaining popularity in other communities, reflective optical systems are now beginning to challenge the established refractive optical systems.
In general, reflective optical systems provide superior performance over refractive optical systems. Reflective optical systems provide superior thermal stability and radiation resistance, and they offer lower image defects arising from chromatic aberration (unlike reflective elements, refractive elements focus different wavelengths of radiation at different focal points).
For certain applications, reflective optical systems can be made far more compact than refractive systems. Reflective systems can operate on a wider range of wavelengths than can refractive optics. A reflective optical systems can operate on both visible and infrared radiation. In contrast, an all-refractive system can operate on visible light or it can operate on infrared radiation, but it cannot operate on both visible and infrared radiation. Thus, an all-reflective surveillance camera would require only a single set of optics for viewing visible and infrared radiation, whereas an all-refractive camera would require two sets of optics: one set for viewing visible radiation, and the other set for viewing infrared radiation. The size and weight savings are impressive and obvious; the elimination of boresight issues is equally impressive, but less obvious.
One type of all-reflective system having a wide range of applications is a three-mirror anastigmat (TMA). The TMA is a re-imaging system, having an objective portion that forms an intermediate image and a relay portion that relays the intermediate image to a plane for viewing. The TMA permits correction of the three fundamental types of geometric aberrations: spherical aberration, coma and astigmatism (three mirrors being the minimum number of elements required for correction of these aberrations in the absence of certain symmetry conditions). The TMA can also be designed to correct for curvature of the field of view.
One such TMA 2 is shown in FIG. 1. The TMA 2 includes a primary mirror 3, a secondary mirror 4, and a tertiary mirror 5. The primary mirror 3 receives optical signals through an entrance pupil 6 and forms an intermediate image 7, which is between the primary mirror 3 and the secondary mirror 4. The secondary mirror 4 and tertiary mirror 5 cooperate to relay the intermediate image through an exit pupil 8 to a focal plane 9 for viewing. This TMA 2 is disclosed in Cook U.S. Pat. No. 4,834,517, issued on May 30, 1989 and assigned to Hughes Aircraft Company, the assignee of this invention. Cook U.S. Pat. No. 4,834,517 is incorporated herein by reference.
The off-axis TMA 2 covers wide fields of view on a flat focal surface at fast optical speeds (optical speed, denoted by an f/number, is proportional to the amount of light collected by the optical system, and it can be calculated as the angle of the F-cone or equivalently as the focal length of the optical system divided by the entrance pupil diameter). For tactical infrared imaging, the off-axis nature of the TMA 2 yields an unobscured aperture, and the relayed nature allows stray radiation to be rejected. The relayed nature of the TMA 2 also allows for 100 percent cold shielding, which is critical for modern tactical infrared detectors.
In addition to the above beneficial characteristics, the TMA 2 has an additional characteristic that can be valued quite highly. Due to the significant angle at which the imaging F-cones intercept the focal plane 9, the TMA 2 can be designed to preclude the reflection of radiation back to its source. This overcomes a problem known as signature augmentation, which is apparent to anyone who has taken a photograph of a person with a camera having its flash bulb mounted directly above the camera's lens: the person in the picture appears to have "red eyes." Signature augmentation occurs because the retina absorbs all but red light from the bulb, and reflects the red light back to the camera lens and onto the film. If the TMA 2 is operated at a small incidence angle, it too will reflect light back to the light source. In certain wide-field applications, this can have serious consequences.
It is apparent from FIG. 1 that the elimination of signature augmentation requires the imaging F-cones to be everywhere outside the normal of the focal plane 9. This necessarily offsets the exit pupil 8 from the focal plane 9, thereby requiring an off-axis cryo-dewar to be built. For those applications where the presence of signature augmentation is of no concern, the cost of the off-axis dewar presents a hardship.
The TMA 2 could accommodate an on-axis dewar (where the cold shield aperture is directly over the focal plane array) if the field of view were offset signficantly. However, such a large field offset would not allow the correction of image aberrations and distortion to the levels generally required for most applications.